The Code Book: The Secret History of Codes and Code-breaking. Simon Singh

7 The science of secret writing and its main branches.

      At first sight, codes seem to offer more security than ciphers, because words are much less vulnerable to frequency analysis than letters. To decipher a monoalphabetic cipher you need only identify the true value of each of the 26 characters, whereas to decipher a code you need to identify the true value of hundreds or even thousands of codewords. However, if we examine codes in more detail, we see that they suffer from two major practical failings when compared with ciphers. First, once the sender and receiver have agreed upon the 26 letters in the cipher alphabet (the key), they can encipher any message, but to achieve the same level of flexibility using a code they would need to go through the painstaking task of defining a codeword for every one of the thousands of possible plaintext words. The codebook would consist of hundreds of pages, and would look something like a dictionary. In other words, compiling a codebook is a major task, and carrying it around is a major inconvenience.

      Second, the consequences of having a codebook captured by the enemy are devastating. Immediately, all the encoded communications would become transparent to the enemy. The senders and receivers would have to go through the painstaking process of having to compile an entirely new codebook, and then this hefty new tome would have to be distributed to everyone in the communications network, which might mean securely transporting it to every ambassador in every state. In comparison, if the enemy succeeds in capturing a cipher key, then it is relatively easy to compile a new cipher alphabet of 26 letters, which can be memorised and easily distributed.

      Even in the sixteenth century, cryptographers appreciated the inherent weaknesses of codes, and instead relied largely on ciphers, or sometimes nomenclators. A nomenclator is a system of encryption that relies on a cipher alphabet, which is used to encrypt the majority of a message, and a limited list of codewords. For example, a nomenclator book might consist of a front page containing the cipher alphabet, and then a second page containing a list of codewords. Despite the addition of codewords, a nomenclator is not much more secure than a straightforward cipher, because the bulk of a message can be deciphered using frequency analysis, and the remaining encoded words can be guessed from the context.

      As well as coping with the introduction of the nomenclator, the best cryptanalysts were also capable of dealing with badly spelt messages and the presence of nulls. In short, they were able to break the majority of encrypted messages. Their skills provided a steady flow of uncovered secrets, which influenced the decisions of their masters and mistresses, thereby affecting Europe’s history at critical moments.

      Nowhere is the impact of cryptanalysis more dramatically illustrated than in the case of Mary Queen of Scots. The outcome of her trial depended wholly on the battle between her codemakers and Queen Elizabeth’s codebreakers. Mary was one of the most significant figures of the sixteenth century – Queen of Scotland, Queen of France, pretender to the English throne – yet her fate would be decided by a slip of paper, the message it bore, and whether or not that message could be deciphered.

      The Babington Plot

      On 24 November 1542, the English forces of Henry VIII demolished the Scottish army at the Battle of Solway Moss. It appeared that Henry was on the verge of conquering Scotland and stealing the crown of King James V. After the battle, the distraught Scottish king suffered a complete mental and physical breakdown, and withdrew to the palace at Falkland. Even the birth of a daughter, Mary, just two weeks later could not revive the ailing king. It was as if he had been waiting for news of an heir so that he could die in peace, safe in the knowledge that he had done his duty. Just a week after Mary’s birth, King James V, still only thirty years old, died. The baby princess had become Mary Queen of Scots.

      Mary was born prematurely, and initially there was considerable concern that she would not survive. Rumours in England suggested that the baby had died, but this was merely wishful thinking at the English court, which was keen to hear any news that might destabilise Scotland. In fact, Mary soon grew strong and healthy, and at the age of nine months, on 9 September 1543, she was crowned in the chapel of Stirling Castle, surrounded by three earls, bearing on her behalf the royal crown, sceptre and sword.

      The fact that Queen Mary was so young offered Scotland a respite from English incursions. It would have been deemed unchivalrous had Henry VIII attempted to invade the country of a recently dead king, now under the rule of an infant queen. Instead, the English king decided on a policy of wooing Mary in the hope of arranging a marriage between her and his son Edward, thereby uniting the two nations under a Tudor sovereign. He began his manoeuvring by releasing the Scottish nobles captured at Solway Moss, on the condition that they campaign in favour of a union with England.

      However, after considering Henry’s offer, the Scottish court rejected it in favour of a marriage to Francis, the dauphin of France. Scotland was choosing to ally itself with a fellow Roman Catholic nation, a decision which pleased Mary’s mother, Mary of Guise, whose own marriage with James V had been intended to cement the relationship between Scotland and France. Mary and Francis were still children, but the plan for the future was that they would eventually marry, and Francis would ascend the throne of France with Mary as his queen, thereby uniting Scotland and France. In the meantime, France would defend Scotland against any English onslaught.

      The promise of protection was reassuring, particularly as Henry VIII had switched from diplomacy to intimidation in order to persuade the Scots that his own son was a more worthy groom for Mary Queen of Scots. His forces committed acts of piracy, destroyed crops, burnt villages and attacked towns and cities along the border. The ‘rough wooing’, as it is known, continued even after Henry’s death in 1547. Under the auspices of his son, King Edward VI (the would-be suitor), the attacks culminated in the Battle of Pinkie Cleugh, in which the Scottish army was routed. As a result of this slaughter it was decided that, for her own safety, Mary should leave for France, beyond the reach of the English threat, where she could prepare for her marriage to Francis. On 7 August 1548, at the age of six, she set sail for the port of Roscoff.

      Mary’s first few years in the French court would be the most idyllic time of her life. She was surrounded by luxury, protected from harm, and she grew to love her future husband, the dauphin. At the age of sixteen they married, and the following year Francis and Mary became King and Queen of France. Everything seemed set for her triumphant return to Scotland, until her husband, who had always suffered from poor health, fell gravely ill. An ear infection that he had nursed since a child had worsened, the inflammation spread towards his brain, and an abscess began to develop. In 1560, within a year of being crowned, Francis was dead and Mary was widowed.

      From this point onwards, Mary’s life would be repeatedly struck by tragedy. She returned to Scotland in 1561, where she discovered a transformed nation. During her long absence Mary had confirmed her Catholic faith, while her Scottish subjects had increasingly moved towards the Protestant church. Mary tolerated the wishes of the majority and at first reigned with relative success, but in 1565 she married her cousin, Henry Stewart, the Earl of Darnley, an act that led to a spiral of decline. Darnley was a vicious and brutal man whose ruthless greed for power lost Mary the loyalty of the Scottish nobles. The following year Mary witnessed for herself the full horror of her husband’s barbaric nature when he murdered David Riccio, her secretary, in front of her. It became clear to everyone that for the sake of Scotland it was necessary to get rid of Darnley. Historians debate whether it was Mary or the Scottish nobles who instigated the plot, but on the night of 9 February 1567, Darnley’s house was blown up and, as he attempted to escape, he was strangled. The only good to come from the marriage was a son and heir, James.

      Mary’s next marriage, to James Hepburn, the Fourth Earl of Bothwell, was hardly more successful. By the summer of 1567 the Protestant Scottish nobles had become completely disillusioned with their Catholic Queen, and they exiled Bothwell and imprisoned Mary, forcing her to abdicate in favour of her fourteen-month-old son, James VI, while her half-brother, the Earl of Moray, acted as regent. The next year, Mary escaped from her prison, gathered an army of six thousand royalists, and made a final attempt to regain her crown. Her soldiers confronted the regent’s army at the small village of Langside, near Glasgow, and Mary witnessed the battle from a nearby hilltop. Although her troops were greater in number, they lacked discipline, and Mary watched as they were torn apart. When defeat was inevitable, she fled. Ideally she